1. Field of the Invention
The invention relates to an implant set having modularity with conformity total knee replacement. More particularly this invention is meant to provide the exact conformity of femoral component (1) and tibial insert (2) and apt combination of femoral component (1) and tibial component (4).
2. Description of the Related Art
A) The total knee replacement Implant set consists of four components, namely, a femoral metal component, a tibial component (4), a tibial Insert (2) and a patella.
B) In some implant sets the tibial component (4) and tibial insert (2) are assembled together by manufacturer as one unit in which the tibial component (4) is made of metal and the insert is made of polyethylene.
C) In some implant sets this tibial component (4) tibial Insert (2) assembly is omitted and is replaced by an all poly tibial component (4) which has same dimension as that of tibial component (4) (metal) and tibial insert (2) (Polyethylene) assembly.
Type ‘B’ is known as a monoblock system.
Type’ is known as an All Poly system.
In each of the above mentioned types there are various sizes available to match with the anatomical sizes of the patient i.e. size goes on increasing in ‘anterior posterior’ dimension and medial-lateral dimension. The size means the size of each component, such as the femoral component (1), tibial component (4) and tibial insert (2).
Thus there would be about 5 to 8 sizes of femoral components (1) and 5 to 8 sizes of tibial components (4) and tibial inserts (2).
Generally, in the prior art it is expected that in the monoblock system femoral component (1) will have corresponding tibial component (4). For example, Femoral components (1) will increase in size from smaller to larger and would be named as size 1, 2, 3, 4, 5, etc. In the same way, tibial components (4) will be named as size 1, 2, 3, 4, 5, etc. Therefore, size 1 of femoral components (1) will match with size 1 of tibial components (4) and so on.
But in day-to-day practice this logic is not suitable. The anatomy of the patient is different because of a number of parameters and conditions. Therefore, Femoral components (1) size 1 will not always match with tibial component (4) of size 1 in A/P and M/L dimensions.
It happens that size 2 of femoral components (1) will require size 1 tibial components (4) or size 3 tibial components (4A). This situation demands a ‘modular system’ of implants which will consist of femoral component (1), tibial component (4) and tibial insert (2) i.e. Type ‘A’ like system.
Currently, nearly all the implant sets are available are ‘modular systems’.
This modularity allows the mix-n-match of the components in allowable limits prescribed by the designer/manufacturer. But this mix-n-match of the components in allowable limits always forces the user to make the compromise in the ‘Conformity’ of the ‘Femoral component’ (1) to tibial Insert (2)/tibial Component (4) assembly.
‘Conformity’ means the dimensional matching of the two articulating geometries i.e. in the current scenario geometrical matching of the articulating surface of femoral components (1) and tibial Insert (2).
It is always preferable to have the ‘exact foot-print’ of femoral component (1) on the tibial insert (2).
In case of ‘mix-n-match’ of the components the compromises are as follows:
The case 1: Referring to FIGS. 4A and 4B, indicate non-conforming dimension on tibial insert (2) and obstructing dimension of femoral component (1);
1) FIGS. 3A and 3B—Exactly conforming;
2) FIGS. 4A and 4B—Non conforming dimensions, e.g. tibial insert (2)/tibial component bigger than femoral component (1);
3) FIGS. 5A and 5B—Femoral component (1) bigger than tibial insert (2)/tibial component (4) assembly;
The case 2: Referring FIGS. 5A and 5B indicate non-conforming geometries of tibial insert (2) loose fitting dimensions on femoral component (1).
Thus, FIGS. 3A and 3B show the perfectly conforming geometry
FIGS. 4A and 4B show the ‘loose’ condylar and obstructing’ notch dimensional fit.
FIGS. 5A and 5B show the ‘loose’ notch and obstructing condylar dimensional fit.
To avoid obstructing post (3) with box, the dimension of the “femoral box’ (13) is increased for smaller size, i.e. femoral component (1) of size 1 will have the same box width as that of size 2.
To reduce the dimensional interference
wf>wp & df>dp,
wf>wp should go on increasing as size increases in relation to each other,
df>dp should go on increasing as size increases in relation to each other.
But because of current available modularity between implants,
1. to accommodate the post (3) of ‘size 2 tibial insert’ (2) in the box of ‘size 1 femur’ the box of size 1 femur must be widened and deepened, resulting in extra resection of bone (4A and 4B); case 1.
2. As in case 2, i.e. FIGS. 5A and 5B, where tibial insert (2) is of smaller size than femoral component (1), there will be excess clearance between femoral box (13) and tibial post (3), resulting in increased stress of the post (3) leading to faster wear of the post (3).
To avoid all these compromises described in case 1 or case 2, the solution is an implant set having modularity with conformity for total knee replacement according to the present invention.